1. Field of the Invention
The present invention relates to a driving circuit used in a digital communication system and, more particularly, relates to a driving circuit effective to control a peak value of output voltage applied to a load in accordance with an impedance value of the load.
2. Description of the Related Arts
In recent years, in order to spread digital communication network, standardization of interfaces between various types of digital devices has been promoted. According to the plan of Integrated Services Digital Network (ISDN), study and discussion for the standardization have been continued in International Consultative Committee for Telephone and Telegraph (CCITT). Particularly, CCITT recommendation I.430 defines the standardized regulation for the electrical characteristics with regard to "S"-interface. FIG. 9 shows the regulatory characteristic between a peak voltage V2 of transmitting signal and a load resistance R.sub.L across a pair of transformer end terminals according to the CCITT recommendation I.430. In this regulation, the peak voltage V2 must be regulated between the hatched portions of FIG. 9 at respective points of R.sub.L =5.6 .OMEGA., R.sub.L =50 .OMEGA. and R.sub.L =400 .OMEGA.. More specifically, when R.sub.L equals to 50 .OMEGA., V2 should be regulated within the range of 0.75 V .+-.10%. When R.sub.L equals to 5.6 .OMEGA., V2 should be regulated below 0.15 V.
In a conventional driving circuit, a transformer was widely used for driving a load. The output voltage across the load may be controlled by its turn ratio of primary and secondary coils or by changing input voltage. However, if those controls are applied to the transformer driver to satisfy the regulation illustrated in FIG. 9, there arise many problems.
For example, when resistance of the load resistor is decreased to 5.6 .OMEGA., the regulation that the transmitted peak voltage should be below 150 mV cannot be satisfied due to the affect of internal resistances R.sub.P and R.sub.S of the primary and secondary coils which cannot be decreased sufficiently. On the other hand, when the load resistance R.sub.L is a relatively large value of 50 .OMEGA. or 400 .OMEGA., there is a drawback that the ringing wave is produced during the transitional response of the transformer.